The measurement of surface tension is important for understanding and controlling physico-chemical behavior at gas-liquid and other interfaces. In industry, typical applications include the formulation of detergents, preparation of emulsions, and concentration of ores by froth flotation.
Surface tension is commonly measured using static, equilibrium techniques such as the duNouy tensiometer. Yet in many applications involving rapid or brief contact of dispersed phases, surface equilibrium is not reached. For such, static measurements may be of little value, since they do not reveal transient or rate-limited phenomena in which surface tension may be changing rapidly, e.g., in emulsification and in ore flotation.
To study these, dynamic surface tension measurement is required. A dynamic procedure frequently used is the maximum bubble pressure method. In it, gas is passed through an orifice tube into the test liquid, forming a succession of bubbles. The inflation pressure inside each bubble is at a maximum when the bubble achieves minimum radius of curvature. This occurs as the bubble assumes hemispherical shape at the orifice. The maximum bubble pressure is thus directly related to, and provides a true measure of, the surface tension of the liquid. (W. J. Moore, Physical Chemistry, 3rd Ed., p. 729-31, Prentice Hall, N.J., 1962.)
This method has been used to study transient surface behavior of aqueous surfactant solutions. For some solutions, the observed value of dynamic surface tension is not constant but changes when the rate of bubbling is altered. The change, which differs from one surfactant to another, has been attributed to diffusion phenomena and used to study them. Tensiometers for making appropriate measurements have been described. (A. M. Kragh, Trans. Faraday Soc. 60 (1), p. 225-232 (1964); U.S. Pat. No. 3,881,344 (1975). Cf. U.S. Pat. No. 3,426,584 (1969).)
Using these instruments, maximum bubble pressure is observed as a value averaged over a number of bubbles generated at a fixed rate. When more than one rate is to be studied, the rate of bubbling must be changed manually, and another maximum pressure average value taken. The averaging makes for insensitivity to small or rapid changes in maximum bubble pressure, and the instruments to not admit of easy, precise control and change of bubbling rate. In addition, the technique is slow and cumbersome for determining changes in maximum bubble pressure over a wide range of bubbling rates.